tile: remove two outdated Kconfig entries

Tile support got added in v2.6.36. Its main Kconfig file was added with
two outdated Kconfig entries. DEFAULT_MIGRATION_COST is unused since
v2.6.23, and SEMAPHORE_SLEEPERS is unused since v2.6.26. Remove these
outdated entries now.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>